Changes to SVG-AAM to support accessible use-element shadow trees

Hello all,

I've drafted some changes to the SVG-AAM to support a model of <use>
element content with fully accessible shadow DOM.

Diffs are here: https://github.com/w3c/aria/compare/svg-aam-shadow-dom

Compiled document is here:
https://rawgit.com/w3c/aria/svg-aam-shadow-dom/svg-aam/svg-aam.html

All of this is conditional on the corresponding changes to SVG 2 being
tidied up & merged.  The relevant pull request for that is here:
https://github.com/w3c/svgwg/pull/206

As I worked through this, I thought of some broader issues that apply to
the accessibility of all shadow DOM content.  I started a thread on the
public-aria list, archived here:
https://lists.w3.org/Archives/Public/public-aria/2016Jul/0146.html

I would of course be happy to talk through this on the call tomorrow, or
next week if people would prefer to have a chance to look at the changes
first.  There are some important consequences for authoring guidance,
especially considering that authors today have to consider both the current
state of affairs and the ideal future state.

Best,
~Amelia